1. Field of Invention
This application relates to processes and systems for making particles, and more particularly processes and systems for making particles having a dimension less than about 1 mm and particles and compositions containing the particles.
2. Discussion of Related Art
The contents of all references, including articles, published patent applications and patents referred to anywhere in this specification are hereby incorporated by reference.
Sub-millimeter particles having custom-designed shapes are highly desirable for a broad range of applications, including making controlled assemblies. However, some existing methods for making custom-shaped particles rely upon a lithographic process (Madou, M. J. Fundamentals of microfabrication: The science of miniaturization. 2nd ed.; CRC Press: Boca Raton, 2002). Typically, these methods require exposing a radiation-sensitive material that has been deposited on a substrate to spatially patterned radiation without degrading the substrate (see e.g. Hernandez, C. J.; Mason, T. G. Colloidal alphabet soup: Monodisperse dispersions of shape-designed LithoParticles. J. Phys. Chem. C, 2007, 111, 4477-4480). (Shape-designed particles, regardless of the methods of production, will also be referred to as LithoParticles in this specification.) In a basic implementation, a polymer resist layer can be cross-linked by the optical exposure and, after development, the polymer resist particles can be released from the substrate (see U.S. application Ser. No. 12/377,773 filed Feb. 17, 2009 as a national stage application of PCT/US07/18365, entitled “Customized Lithographic Particles,” by the same assignee as the current application, the entire contents of which are hereby incorporated by reference). In some cases, the lithographic device that produces the spatially patterned radiation is very expensive to purchase and maintain. Therefore, developing a method of producing sub-millimeter particles without having to rely upon a complex exposure system to make repeated exposures in order to mass-produce sub-millimeter particles is highly desirable.
One method of producing particles that does not involve mechanical imprinting is relief deposition templating. In this method, a patterned relief surface is created on a solid substrate, and a deposition of a particle material is made in a manner that creates discrete regions that can be separated from the template and retain a geometrical feature imparted by the template. Two implementations of this are pillar deposition templating (Hernandez, C. J.; Zhao, K.; Mason, T. G. Pillar-deposition particle templating: A high-throughput synthetic route for producing LithoParticles (Soft Materials 2007, 5, 1-11)) in which the particles are formed on the top surfaces of pillars (i.e. relief projections), and well deposition templating (Hernandez, C. J.; Zhao, K.; Mason, T. G. Well-deposition particle templating: Rapid mass-production of LithoParticles without mechanical imprinting (Soft Materials 2007, 5, 13-31)), in which the particles are formed by wells (i.e. relief depressions) in the template. Relief deposition templating (U.S. application Ser. No. 12/563,907 titled “Mechanical Process for Creating Particles in a Fluid” filed Sep. 21, 2009 as a CIP of PCT/U.S.08/03679) offers several advantages over mechanical imprinting, since a deposition of a particle material onto a single patterned surface to produce particles is typically much simpler than using a more complex mechanical imprinting device (see e.g. Chou, S. Y. Nanoimprint lithography and lithographically induced self assembly. MRS Bulletin 2001, 26, 512; Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint lithography. J. Vacuum Sci. Tech. B 1996, 14 (6), 4129-4133; Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C G Improved step and flash imprint lithography templates for nanofabrication. Microelectronic Engineering 2003, 69, 412-419). However, one aspect of the method of relief deposition templating is that typically a portion of the shapes of the particles is not defined by the template, but instead is defined by properties of the particle material and by the method of deposition. This aspect can be an advantage in some cases for certain applications of custom-shaped particles, but it can also be a disadvantage in others for which precise specification of the exact shape of the entire surfaces of the produced particles is desired. Moreover, the range of particle shapes that can be produced using only a single patterned surface is limited. Therefore, it would be highly desirable to develop an alternative method for making a wide variety of particle shapes that overcome these existing limitations.
Mechanical imprinting, whether thermal (see e.g. Chou, S. Y. Nanoimprint lithography and lithographically induced self assembly. MRS Bulletin 2001, 26, 512; Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Nanoimprint lithography. J. Vacuum Sci. Tech. B 1996, 14 (6), 4129-4133) or step-and-flash (see e.g. Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C G Improved step and flash imprint lithography templates for nanofabrication. Microelectronic Engineering 2003, 69, 412-419), is a technology that involves bringing two solid surfaces into contact after depositing a desired material between them. Once the surfaces of the two plates touch, the material only fills trenches or wells in one plate that has been prepared with the desired relief patterns in the surfaces of the plates. Imprinting essentially forces a desired material into void-like regions that have been created in one of the surfaces to form a mold. While the two plates are touching (or nearly touching), a process, such as cross-linking in the case of polymers, can be used to rigidify the material in the mold, and then the plates are separated. During the separation, if the release of the desired material from the corrugated surface can be made efficiently, then the result is a set of raised structures of the desired material on the flat surface of the other plate. Imprinting is a subset of the more general process of embossing, in which a mold is pressed into the surface of a material that is not as rigid and then removed to create raised corrugations that reflect the mold. However, by contrast to embossing, mechanical imprinting involves squeezing out material between two solid plates where they touch, so that only the negative relief corrugations in one plate become filled with the desired material.
Although it is possible to create particles using a single patterned plate imprinting method (see e.g. Rolland, J. P.; Maynor, B. W.; Euliss, L. E.; Exner, A. E.; Denison, G. M.; DeSimone, J. M. Direct fabrication of monodisperse shape-specific nanobiomaterials through imprinting (J. Am. Chem. Soc. 2005, 127, 10096-10100)), this method lacks the basic capacity to produce a wide variety of particles shapes having topologies that would preclude release of the particle material from the imprinting template. In particular, by practicing the method of Rolland et al., it is not possible to produce the widest possible variety of particle shapes because certain shapes certain shapes that are unsuitable for release from a single patterned plate would become trapped in the patterned plate after the particle material is solidified and therefore could not be subsequently released from the patterned plate. Moreover, Rolland et al. teach only a method of imprinting particles that involves a first patterned plate and a smooth plate, so the relative alignment of the patterned plate relative to the smooth plate is inconsequential and does not affect the shapes of the particles produced. Thus, in the imprinting process described by Rolland et al., the contours of a single patterned plate and a flat smooth plate dictate the shapes of the particles that are produced. Also, the method of Rolland et al. is limited to producing only a subset of all possible shapes because only a limited range of shapes can be released from a single patterned plate after solidification of a particle material. For example, certain particle shapes would get stuck in the depressions in the patterned plate after solidification and could not be easily removed. As another example, it would not be possible to produce and release a spherical particle through the process involving a single patterned plate described by Rolland et al. Moreover, in the single-patterned-plate method of Rolland et al., the shapes of particles produced do not depend on the relative alignment and orientation of the patterned plate and the smooth plate; instead, the shapes of particles produced depend only on the shapes of depressions in the patterned plate. These limitations of the method of Rolland et al. are significant, and there is a need to improve methods of making particles beyond the current art to overcome these inherent limitations.
By contrast to the existing methods of making custom-shaped particles, it would be highly desirable to create a method of making particles for which the shape of the resulting particles is actually controlled in a desired and pre-specified manner by the relative alignment of a first patterned surface with respect to a second patterned surface, as well as by local surface relief features, whether positive or negative in the first and second patterned surfaces.
Another existing method of creating custom-shaped particles is relief radiation templating (U.S. application Ser. No. 12/575,920 titled “Process for Creating Shape-Designed Particles in a Fluid” filed Oct. 8, 2009). In this method, the shape of the particle is designed and formed by a combination of patterned surface relief features on a relief template and exposure to spatially patterned radiation. This combination provides shape-designed particles that can have more complex shapes than simple relief deposition templating, at the cost of requiring repeated use of the lithographic exposure system. In the relief radiation templating method, as with the relief deposition templating method, a portion of the shape of the particle may result from a deposition process and may not be pre-specified by either the patterned surface relief features or by the spatially patterned radiation. The precision of certain surface features on particles that can be obtained by certain deposition processes may not be suitable for certain applications of custom-shaped particles.
Consequently, there remains a need for improved processes and systems for making shape-designed particles having a dimension less than about 1 mm.